Someone who's mostly straight or gay, with one exception. It's often separated into three categories:
Sole Exception: The character in question has a one-off attraction to that person that transcends their
sexual preference. This is based on mutual compatibility, or otherwise
emotional attraction(this is similar to Demisexuality).
Even the Gays/Straights Want Them: The target of attraction is presented as being physically attractive enough to transcend the sexual preferences of people, not just the attracted character.
Closet Key: The exception gradually becomes the rule for that character (this not being a subversion as such as the trope applies at the beginning of the
relationship and is seen as responsible for the gradual reveal of the character's sexuality/gradual transition of their gender preference).